In image coding techniques such as MPEG2, H. 264, and the like, an intra coding process and an inter coding process are used. In the intra coding process, coding is completed within a frame to be coded, without using any reference frames. In the inter coding process, coding for a frame to be coded is performed by using one or more reference frames.
All macroblocks included in an I picture are coded by the intra coding process. Macroblocks included in a P picture and a B picture are coded by the inter coding process. Since all the macroblocks in the I picture are intra-coded, the amount of generated codes in the I picture is larger than that in the P picture or the B picture. Even in the H.264 where an intra prediction technique is used, the amount of generated codes in the I picture is still larger than that in the P picture or the B picture.
In a system in which low-delay reproduction or transmission is required, an increase in the amount of generated codes in the I picture becomes a problem. Further, there is another problem that the amount of generated codes oscillates in a cycle of appearance of the I picture. When the amount of generated codes oscillates, a larger buffer size is temporarily needed, which is inefficient.